Simultaneous transmit and receive (STAR) communication systems transmit radio-frequency (RF) (including microwave) signals at the same time, or nearly the same time, they receive RF signals. The transmitted signals are of the same frequency, or within a relatively narrow frequency band, as the received signals. In some cases, such as in a frequency-hopping system, the transmit and receive frequencies may be harmonically related. A STAR system includes a local transmitter and a local receiver. The STAR system may utilize separate transmit and receive antennas, or a single antenna may be utilized for both transmitting and receiving. A STAR system may communicate with a distant communication system having its own transmitter and receiver, or signals from the STAR system may be transmitted so as to be reflected by a distant object back to the STAR system, such as in a radar system.
Absent special circuits, signals from the local transmitter can overwhelm wanted signals from the distant transmitter or the radar signals reflected by the distant object. If separate transmit and receive antennas are used, signals radiated by the transmit antenna may be directly received, i.e., not via a reflection from another object, by the receive antenna and fed to the local receiver. Similarly, signals radiated by the transmit antenna may be reflected (“backscattered”) by other antenna elements, mounting hardware or other nearby objects (such as terrain, trees, etc.), other than the object being tracked by radar, and received by the receive antenna and fed to the local receiver. If one antenna is used for both transmitting and receiving, transmitter signals may be reflected by the antenna back down a transmission line, or by other components along the transmission line, to the receiver. Whether or not separate transmit and receive antennas are used, transmitter signals may also undesirably couple into receiver circuits, such as via parasitic capacitances.
Some systems, such as some radar systems, transmit closely spaced-in-time chirps and turn off their receivers during the transmissions. Even in such a system, transmitter signals reflected close to the system may arrive at the receiver while the receiver is on.
Unwanted signals from the local transmitter are referred to as self-interference. Self-interference is typically many dB stronger than wanted received signals. Consequently, self-interference signals can desensitize the local receiver and/or mask the wanted received signals and, therefore, need to be attenuated or completely suppressed.
If one antenna is used for both transmitting and receiving, a circulator is conventionally used as a duplexer to isolate the local receiver from the local transmitter. A circulator is a passive non-reciprocal three- or four-port device, in which a radio frequency signal entering any port is transmitted only, or largely, to the next port in rotation. A typical circulator includes a magnetically biased ferrite disk with three ports coupled to it, creating a Y-junction.
A signal applied to the ferrite disk generates two equal, circularly polarized, counter-rotating waves that rotate at velocities ω+ and ω−. The velocity of a circularly polarized wave as it propagates through the magnetically biased ferrite material depends on its direction of rotation. By selecting a ferrite material and the biasing magnetic field, the phase velocity of the wave traveling in one direction can be made greater than the phase velocity of the wave traveling in the opposite direction.
If a signal is applied at the first port, the two waves arrive in phase at the second port, but they arrive out of phase and destructively interfere with each other at the third port. Maximum power transfer occurs, therefore, from the first port to the second port, and minimum power transfer occurs from the first port to the third port, depending on the direction of the applied magnetic field. Due to the symmetry of the Y-junction, similar results can be obtained for other port combinations. Externally, the circulator appears to direct signal flow clockwise or counterclockwise, depending on the polarization of the magnetic biasing field.
However, because circulators rely on distances between ports being related to wavelength, circulators have narrow bandwidths. Furthermore, although circulators can largely isolate a receiver port from a transmitter port, they do not isolate the receiver port from unwanted signals received via an antenna port. For example, transmitter signals reflected by an antenna or by portions of a transmission line are passed by the circulator to the receiver, as though they were wanted signals. Similarly, transmitter signals reflected by other antennas, mounting hardware and other nearby objects and received by the antenna are passed by the circulator to the receiver, as though they were wanted signals. In addition, ferrite circulators are characterized by relatively high insertion loss and, in some cases being nonlinear devices, they can generate undesirable harmonics.